1. Field of the Invention
This invention relates to a method for correcting image density in thermo-optic recording, wherein the image density of an image recorded on a thermosensitive recording material is prevented from fluctuating due to various factors concerning the image recording in an apparatus for scanning the thermosensitive recording material with a laser beam modulated in accordance with an image signal and thereby recording the image thereon.
2. Description of the Prior Art
Optical scanning recording apparatuses have heretofore been used wherein a laser beam is modulated in accordance with an image signal, a photosensitive material is scanned with the modulated laser beam, and an image represented by the image signal is thereby recorded on the photosensitive material.
Recently, thermo-optic recording apparatuses (i.e. thermal types of optical scanning recording apparatuses) have been proposed wherein a laser beam is utilized as a heat source, thermal energy of the laser beam is given to a thermosensitive recording material, and an image is thereby recorded on the thermosensitive recording material.
In the thermo-optic recording apparatuses, a laser is utilized in the heat mode. They have the advantages in that a visible image can be obtained with thermal energy simultaneously with the input of a signal and in that additional developing and fixing operations are not required. Such apparatuses are disclosed in, for example, Japanese Unexamined Patent Publication Nos. 50(1975)-23617, 58(1983)-94494, 62(1987)-77983, and 62(1987)-78964.
However, with the thermo-optic recording apparatuses described above, a thermosensitive color forming layer of the thermosensitive recording material does not easily absorb light having wavelengths falling within the wavelength distribution ranges of visible light and infrared rays, and therefore the output power of the laser must be made very large. Accordingly, it was difficult for the thermo-optic recording apparatuses described above to be made small in size and low in cost.
In order to solve the aforesaid problems, the applicant proposed a novel thermosensitive recording material which is capable of recording good images having good image quality. The proposed thermosensitive recording material comprises a substrate and a color forming agent, a developing agent, and a light absorbing dye, which are overlaid on the substrate. The proposed thermosensitive recording material forms a color having a density according to the level of thermal energy applied thereto. The applicant also proposed an apparatus for recording an image on the thermosensitive recording material by using a laser beam. The novel thermosensitive recording material and the apparatus for recording an image thereon are proposed in Japanese Patent Application No. 3(1991)-62684 and Japanese Unexamined Patent publication No. 5(1993)-24219.
As proposed in Japanese Patent Application No. 3(1991)-62684, the thermosensitive recording material comprises the substrate and a thermosensitive recording layer overlaid on the substrate. The thermosensitive recording layer is formed by applying a coating composition to the substrate. The coating composition contains an emulsion, which is prepared by dissolving microcapsules containing at least a basic dye precursor, a developing agent, and a light absorbing dye into an organic solvent that is sparingly soluble or insoluble in water, and thereafter emulsifying and dispersing the dissolved materials.
The basic dye precursor has properties such that it may form a color by donating electrons or accepting protons from an acid, or the like. Ordinarily, the basic dye precursor comprises a compound, which is approximately colorless and has a partial skeleton of lactone, lactam, sultone, spiropyran, ester, amide, or the like, that can undergo ring opening or cleavage when being brought into contact with the developing agent. Specifically, the basic dye precursor may be crystal violet lactone, benzoyl leucomethylene blue, malachite green lactone, rhodamine B lactam, 1,3,3-trimethyl-6'-ethyl -8'-butoxyindolino-benzospiropyran, or the like.
As the developing agent for the color forming agent, an acid substance, such as a phenolic compound, an organic acid or its metal salt, or a hydroxybenzoic acid ester, may be used. The developing agent should preferably have a melting point falling within the range of 50.degree. C. to 250.degree. C. In particular, a phenol or an organic acid, which is sparingly soluble in water and has a melting point falling within the range of 60.degree. C. to 200.degree. C., is desirable as the developing agent. Examples of the developing agents are disclosed in, for example, Japanese Unexamined Patent Publication No. 61(1986)-291183.
The light absorbing dye should preferably be selected from dyes, which absorb less light having wavelengths falling within the wavelength distribution range of visible light and have a particularly high absorptivity with respect to light having wavelengths falling within the wavelength distribution range of infrared rays. Examples of such dyes include cyanine dyes, phthalocyanine dyes, pyrylium and thiopyrylium dyes, azulenium dyes, squarylium dyes, metal complex dyes containing Ni, Cr, or the like, naphthoquinone and anthraquinone dyes, indophenol dyes, indoaniline dyes, triphenylmethane dyes, triallylmethane dyes, aminium and diimmonium dyes, and nitroso compounds. Among the above-enumerated dyes, the dyes which have a high absorptivity with respect to light having wavelengths falling within the range of 700nm to 900nm in the near infrared region are more preferable. This is because semiconductor lasers capable of producing near infrared laser beams have been put to practical use.
However, in the thermo-optic recording apparatuses described above, even if the image signal is the same, the image density of the image recorded with a laser beam modulated in accordance with the image signal will vary due to, for example, fluctuations in the diameter of the laser beam produced by a laser beam source, such as a semiconductor laser, and a difference between thermosensitive recording materials.
In particular, if the image density fluctuates in the manner described above during the operations for recording images for medical purposes, which images are required to have particularly high gradation, the image quality of the images will become worse and the images cannot serve as an effective tool in the efficient and accurate diagnosis of an illness.
A method for correcting image density in an image recording operation is disclosed in, for example, Japanese Unexamined Patent Publication No. 4(1992)147870. With the disclosed method, a thermal head is used as a heating means, and an image is recorded on a recording material by driving the thermal head. Before the image is recorded on the recording material, a plurality of density patterns are recorded on the forward side, which is taken in the direction along which the thermal head passes. Thereafter, the densities of the recorded density patterns are measured, and a correction is made in accordance with the results of measurements of the densities such that the image which is then recorded on the recording material may have an appropriate level of image density.
However, as compared with the recording method utilizing the scanning with a laser beam, the thermal recording method utilizing a thermal head is disadvantageous in that the recording speed and the resolution of the recorded image are lower. Therefore, the recording method utilizing a laser beam is practically more useful.